1. Field of the Invention
The present invention relates to an optical pickup device including a first laser source which emits laser light having a first wavelength, and a second laser source which is arranged in an optical path apart from that of the first laser source and emits laser light having a second wavelength which differs from the first wavelength. In this optical pickup device, each of the laser light rays emitted from the first and the second laser source is introduced into a common optical path by means of a polarizing beam splitter. The laser light directed into the common optical path is reflected by a plate-like beam splitter so as to be directed through an objective lens. The laser light is then converged by means of the objective lens and irradiated on a signal recording medium. Laser light reflected off of the signal recording medium is transmitted through the plate-like beam splitter and directed to an optical detector.
2. Description of the Related Art
Optical pickup devices which are compatible for performing recording and reproduction using signal recording mediums having different recording densities such as DVDs (digital versatile discs) and CDs (compact discs) have been known. A conventional optical pickup device compatible for performing recording and reproduction using DVDs and CDs may be configured using a beam splitter. In such a device, a laser source for DVD and a laser source for CD may be respectively arranged on the reflecting side and on the transmitting side of the splitting surface. Alternatively, in an opposite approach, the laser source for CD and the laser source for DVD may be respectively arranged on the reflecting side and the transmitting side. The optical pickup device is configured such that laser light emitted from each of the laser sources is directed into a common optical path by means of the beam splitter and irradiated into an objective lens. By selecting either of the laser sources depending on the type of the disc used, the optical pickup device can execute recording and reproduction using DVDs and CDs.
When employing a beam splitter as described above in order to arrange the laser source for DVD and the laser source for CD in optical paths apart from one another, a polarizing beam splitter is used as the beam splitter. In this case, in accordance with the typical film characteristic of the splitting surface of the polarizing beam splitter, i.e., the characteristic that p-polarization transmittance is higher than s-polarization transmittance, it is configured such that laser light incident into the reflecting side of the splitting surface of the polarizing beam splitter is s-polarized, while laser light incident into the transmitting side of the splitting surface of the polarizing beam splitter is p-polarized (refer to Japanese Patent Laid-Open Publication No. 2003-141769).
A laser diode used as the laser source for DVD or the laser source for CD emits laser light that is linearly polarized parallel to the p-n junction surface of the laser chip. Accordingly, the polarization direction of the laser light with respect to the splitting surface of the polarizing beam splitter can be set by selecting the orientation (rotating direction) of the laser source for DVD or the laser source for CD.
Further, in a typical optical pickup device, in addition to arranging a laser diode which emits s-polarized laser light on the reflecting side of the splitting surface of the polarizing beam splitter and arranging a laser diode which emits p-polarized laser light on the transmitting side of the splitting surface of the polarizing beam splitter, a quarter-wave plate is provided on the upstream side of the objective lens in the outgoing optical path by which the laser light advances toward the disc. By causing the linear polarization directions of the incoming and outgoing laser light beams passing through the quarter-wave plate to differ by 90°, the film characteristic of the splitting surface of the polarizing beam splitter can be employed to prevent the returning light reflected off of the disc from reaching back to the laser diode emitting laser light.
As described in the above-noted Japanese Patent Laid-Open Publication No. 2003-141769, in order to achieve downsizing and efficient arrangement of various optical elements in an optical pickup device, a light-receiving optical system and a light-emitting optical system may be separately provided by a second beam splitter. The second beam splitter is provided apart from the polarizing beam splitter which is provided in the common optical path for DVD and CD. The light-receiving optical system introduces the returning light reflected off of the disc into the optical detector. The light-emitting optical system for laser light emission includes the polarizing beam splitter, the laser diode for DVD, and the laser diode for CD.
In an optical pickup device in which a second beam splitter is provided and laser light reflected by the second beam splitter is directed through the objective lens, both laser light beams for DVD and CD are reflected by the second beam splitter. Accordingly Light use efficiency in connection with the splitting surface (reflecting surface) of the second beam splitter is higher for s-polarized laser light than for p-polarized laser light.
As described in the above-noted Japanese Patent Laid-Open Publication No. 2003-141769, an optical pickup device compatible for performing recording and reproduction using DVDs and CDs is often designed while giving priority to light use efficiency of the DVD optical system over light use efficiency of the CD optical system, because the emission output intensity obtained using the laser diode of the laser source for DVD often does not include much margin in view of the output intensity determined during the design stage.
In order to prioritize light use efficiency of the laser light for DVD, the laser diode for DVD is arranged on the reflecting side of the polarizing beam splitter such that laser light emitted from the laser diode for DVD is subjected to s-polarization reflection at the splitting surface of the polarizing beam splitter, while the laser diode for CD is arranged on the transmitting side of the polarizing beam splitter such that laser light emitted from the laser diode for CD is subjected to p-polarization transmission at the splitting surface of the polarizing beam splitter.
When, apart from the polarizing beam splitter for separately arranging the first and the second laser sources, the second beam splitter is provided in the common optical path in order to separate the light-receiving optical system for introducing light into the optical detector and the light-emitting optical system for emitting laser light, the polarized states of the laser light for DVD and the laser light for CD with respect to the splitting surface of the second beam splitter are s-polarized and p-polarized, respectively, or vice versa. Accordingly, at the second beam splitter, when the device design is determined while giving priority to light use efficiency of one of the laser lights (laser light for DVD, for example), light use efficiency of the other laser light (laser light for CD, for example) is compromised.
The optical pickup device described in the above-noted Japanese Patent Laid-Open Publication No. 2003-141769 is configured such that both the laser light for DVD and the laser light for CD are reflected at the splitting surface of the second beam splitter so as to be directed to the objective lens. The laser light incident on the splitting surface of the second beam splitter in s-polarized state, i.e., the laser light for DVD whose light use efficiency is given priority, is reflected efficiently, while reflection of the laser light incident on the splitting surface in p-polarized state, i.e., the laser light for CD, is not efficiently performed.
In order to improve the reflection efficiency of the laser light for CD which is incident on the splitting surface of the second beam splitter in p-polarized state in the above-described optical pickup device, it is necessary to implement a further feature for the film design of the splitting surface by, for example, arranging the second beam splitter at an angle such that the angle of the splitting surface with respect to the optical axis of laser light is 30 degrees, which is smaller than 45 degrees. This results in increased limitations to freedom in design, as well as cost disadvantages.
Further, the optical axis on the reflecting side of the polarizing beam splitter is more strongly influenced by a tilt of the polarizing beam splitter as compared to the optical axis on the transmitting side of the polarizing beam splitter. Accordingly, when high importance is placed on light use efficiency of the DVD optical system and therefore the laser diode for DVD is arranged on the reflecting side of the polarizing beam splitter, an optical axis deviation due to a tilt of the polarizing beam splitter would cause problems in the DVD optical system which requires higher accuracy in signal recording density as compared to the CD optical system.